The present invention relates to electroless plating methods and to solutions used in the electroless plating of non-conductive or dielectric surfaces.
Chemical plating solutions for depositing metals by autocatalytic chemical reduction of metal ions in solution and in contact with a catalytic surface of the article to be plated are well known. Such solutions, which do not use electricity, are referred to in the art as electroless plating solutions. Electroless metal depositions is also distinguished from displacement metal plating of a type described in Metals Finishing Guide Book, 27th Ed., 1959, pp. 569 Et Seg., and Metal Mirror Procedures. Electroless metal plating has found particular use in plating non-metallic substrates such as ceramics and plastics.
Typically, commercial prior art electroless metal plating of non-metallic substrates is accomplished by the steps of treating the substrates either with a sensitizing solution containing stannous chloride followed by treatment with an activating solution containing palladium chloride or other noble metal, or by first treating the substrate with a seeding solution containing both stannous chloride and palladium chloride together with a stabilizer followed by a treatment with an accelerator solution.
In addition to the attendant costs involved in a process of this nature due to the high cost of palladium, the high catalytic activity of the noble metals is believed to cause contamination of the electroless plating bath, and thus its decompositions.
Imaging processes using non-noble metal systems are described in U.S. Pat. No. 2,504,593, issued Apr. 18, 1950. In the processes described, a non-metallic substrate is coated with a solution containing ions of a non-noble metal, e.g., copper ions and minor amounts of a light sensitive reducing agent and a non-light sensitive reducing agent. The substrate is then exposed in selected areas to ultra-violet light to produce real images.
Similar processes in which non-noble metals are employed have been described more recently in U.S. Pat. Nos. 3,772,056 and 3,772,078. In the processes described therein, the surface of a non-conductive substrate is coated with an aqueous solution of non-noble metal, dried at elevated temperatures, and then exposed to a reducing agent which reduces the non-noble metal to its metallic state. From a commercial standpoint, however, processes of this nature are less desirable in that only limited adhesion results prior to drying and baking. In fact, it is taught in these patents that the surfaces should be dried before treatment with the reducing agent and that the material will wash off of the surface if it is not dried.
The present invention relates to an improved process for the electroless plating of substrates, especially dielectric or non-metallic substrates which is compatible with present-day commercial procedures and equipment, and which does not require the use of palladium or other noble metals, and specifically includes the step of priming of the substrates with solutions which exhibit strong adhesion and which may be rinsed without the necessity of a drying step, and yet retaining an adsorbed layer which constitutes the basis for the catalytic layer.